In X-ray imaging systems, an X-ray field is projected through a subject and onto a receptor, the receptor either being a chemically coated X-ray sensitive film or an electronic receptor. In recent years, there has been a trend towards increasing use of electronic receptors.
The outer boundary of the X-ray field must be approximately aligned with the outer perimeter of the receptor in order to maximize the field of view of the image by projecting the X-ray field onto the entire receptor, without projecting the X-ray field outside of the outer perimeter of the receptor which could cause unnecessary patient exposure to the X-ray field.
However it is difficult to align the X-ray field with the receptor because the X-ray field is invisible to the naked human eye. Accordingly, conventional X-ray imaging systems project a field of visible light that is aligned with the X-ray field. The field of visible light acts as a proxy of the location of the X-ray field to an X-ray technician who positions the X-ray projector, the subject, and the receptor before the X-ray field is projected. In essence, the light field mimics the position and outline of the X-ray field. The visible light field is projected onto the receptor before the X-ray field is projected to verify that the X-ray field will be projected into the receptor during X-ray imaging. The visible light field is also known as a localizing light.
The importance of aligning the X-ray field with the receptor is well accepted. Nonetheless, if the X-ray field is not aligned or coincident with the visible light field, then when the visible light field is aligned with the receptor, the X-ray field will not be aligned with the receptor. In fact, when the light field and the X-ray field are misaligned to an extent, and the visible light field is aligned with the receptor, the result is that the X-ray field will be misaligned with receptor to that extent.
Accordingly, the alignment or coincidence of the visible light field and the X-ray field must be tested and realigned until the visible light field and the X-ray are aligned within prescribed tolerances. In one conventional technique of testing the alignment, an auxiliary film image receptor with a field of view exceeding that of the primary image receptor is used to delineate the X-ray field. The auxiliary image receptor most commonly used is an X-ray sensitive film or a film in a cassette with a scintillator that is X-ray sensitive and emits light to expose the film. By simultaneously imaging common objects on both image receptors, measurements of the X-ray field size made from the auxiliary image receptor can be referenced to the primary one and the degree of alignment is determined.
The process of aligning the visible light field and the X-ray field using auxiliary film image receptors includes the time to remove the auxiliary film image receptors, develop the film and measure the deviation between the visible light field and the X-ray field on the developed film. This time increases the amount of time that the expensive imaging system is not available for use, which in turn increases the operating cost of the imaging system. In addition, the measurement of the deviation is somewhat inconvenient.
Furthermore, the use of such auxiliary film image receptors is becoming increasingly difficult as X-ray imaging equipment replaces film-based image receptors with digital image displays which in turn diminishes the availability of film, film cassettes, and film processors.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art to reduce the operating cost of X-ray imaging systems by decreasing the amount of time in aligning the visible light field and the X-ray field in an X-ray imaging system. In addition, there is also a need in the art to eliminate the use of film in aligning the visible light field and the X-ray field in an X-ray imaging system.